The present invention relates generally to vehicle brake control and, more particularly, to a system for and a method of controlling longitudinal vehicle braking on slippery surfaces and inclines.
It is well known in the industry that optimum vehicle brake pressure is heavily dependent on road surface conditions, among other factors. However, the ever-changing road surface conditions are almost impossible to continuously monitor and measure, and even harder to apply to vehicle control systems which adjust vehicle braking and traction. Consequently, vehicle braking control devices typically adjust braking based on arbitrary fixed targets which, since fixed, are at best a compromise for braking control on the various driving surfaces such as loose snow, packed snow, rain, wet leaves, and dry pavement.
An example of a typical brake control system is disclosed in U.S. Pat. No. 4,763,260 issued to Sakuma et al. The Sakuma et al reference discloses a brake control system which provides an empirical estimation of the road surface coefficient of friction. The estimations and calculations of Sakuma et al assume uniform, unchanging, flat road surface conditions. However, the changing grades and conditions of a road surface have a direct impact on the braking efficiency.
Another reference, U.S. Pat. No. 4,188,075 issued to Rajput et al, discloses an acceleration system for anti skid brake control systems which controls the amount of air pressure being delivered to the vehicle brake system. The adaptive acceleration system adapts to the coefficient of friction between the road surface and tires of a vehicle to provide variable control of the reapplication of brake pressure during spin up or acceleration of the wheels, such as after a skid has occurred.
In a co-pending, commonly assigned patent, Ser. No. 07/535,678, filed June 11, 1990, external changes are estimated and made available to a variety of control devices in a vehicle. The estimating device estimates external conditions, most notably the coefficient of friction between a vehicle tire and a roadway, and the slope or grade of the hill the vehicle is attempting to climb or descend. From values of driven wheel spin of the vehicle, non-driven wheel spin of the vehicle, brake pressure, and wheel torque, the estimating device infers the value of the coefficient of friction and the hill slope. Instead of being imbedded in a brake control device or a traction control device, the estimator is a separate device which is capable of providing the determined values to a variety of vehicle control applications.
It is seen then that there is a need for a device which can utilize estimated external values of the coefficient of friction and hill slope in a feedforward manner to control vehicle braking on slippery or changing surfaces and hills to compensate for external disturbances in an optimal manner.